1. Field of the Invention
The present invention relates to a semiconductor laser element, a method of fabrication thereof, and a multi-wavelength monolithic semiconductor laser device.
2. Related Background Art
It is becoming popular to develop an integrated optical unit assembled from a semiconductor laser of a wavelength in the 650-nm band for digital versatile disks (DVDs) and another semiconductor laser of a wavelength in the 780-nm band for CD-ROMs. The development is also proceeding of a two-wavelength monolithic semiconductor laser device in which these two lasers are formed monolithically on the same substrate. To ensure that this two-wavelength monolithic semiconductor laser device can be used for both CDs and DVDs, it is necessary to reduce noise by producing a longitudinal multi-mode. To achieve this multi-mode operation, a method of providing an external oscillation circuit has been used. However, there has recently been progress in the development of self-sustained pulsation lasers that can achieve multi-modes by self-sustained pulsation, without an external oscillation circuit, as disclosed in Japanese Patent Application Laid-Open No. 6-13709, for example.
A prior-art example of a two-wavelength monolithic semiconductor laser device B that is capable of self-sustained pulsation is shown in FIG. 8. A laser element C on the left side of this figure is an element for CDs in which an active layer 3 is formed of an AlGaAs compound material and a laser element D on the right side of the figure is a laser element D for DVDs in which an active layer 24 is formed of an InGaAlP compound material.
The laser element C for CDs on the left side of the figure is formed of an n-type clad layer 2 of Al0.4Ga0.6As, the active layer 3 of Al0.12Ga0.88As, and a p-type first clad layer 4 of Al0.4Ga0.6As, in sequence on an n-type GaAs substrate 1. A p-type second clad layer 5 is formed of Al0.4Ga0.6As in the shape of a stripe (a ridge shape) on part of this p-type first clad layer 4. The cross-sectional surface of this ridge-shaped p-type second clad layer (ridge portion) 5 is a quadrilateral such that the width of the upper edge is less than the width of the lower edge, as shown in FIG. 8. This p-type second clad layer 5 is provided to a thickness of 1 μm in order to efficiently confine light into the active layer 3. To prevent the generation of high-order modes, the width thereof is no more than approximately 4 μm. A p-type contact layer 6 is formed of GaAs on this ridge portion 5.
The laser element D for DVDs on the right side of the figure, on the other hand, is formed of a buffer layer 21 of n-type GaAs, an n-type clad layer 22 of In0.5(Ga0.3Al0.7)0.5P, an n-side guide layer 23 of In0.5(Ga0.5Al0.5)0.5P, the active layer 24 of a multiple quantum well (MQW) structure of InGaP/InGaAlP, a p-side guide layer 25 of In0.5(Ga0.5Al0.5)0.5P, a p-type first clad layer 26 of In0.5(Ga0.3Al0.7)0.5P, and an etching-stopping layer 27 of p-type In0.5Ga0.5P, in sequence on the same n-type GaAs substrate 1. A p-type second clad layer 28 is formed of p-type In0.5(Ga0.3Al0.7)0.5P in the shape of a ridge on part of this etching-stopping layer 27. This ridge-shaped p-type second clad layer 28 has an Al composition of greater than 0.7 and thus a larger band gap, in order to efficiently seal light into the active layer 24. The p-type second clad layer 28 is provided to a thickness of 1 μm in order to efficiently confine light into the active layer 24. To prevent the generation of high-order modes, the width thereof is no more than approximately 4 μm. A p-type contact layer 30 is formed of GaAs on this ridge-shaped p-type second clad layer 28.
The ridge portions 5 and 28 of the elements C and D on either side of FIG. 8 are covered by SiO2 films 29C and 29D. A p-side electrode 42 and an n-side electrode 41 are formed at the top and bottom of both of the elements C and D, and a separation groove 43 is formed between the two elements C and D. Currents are injected to the active layers 3 and 24 of the respective elements C and D from the n-side electrode 41 and the p-side electrodes 42. A laser beam of the 780-nm band is emitted from the vicinity of the active layer 3 of the semiconductor laser element C for CDs on the left side of the figure and a laser beam of the 650-nm band is emitted from the active layer 24 of the semiconductor laser element D for DVDs on the right side of the figure.
The method of fabricating the device B is given below. First of all, a stack of the layers 2 to 6 is formed over the entire surface of the n-type GaAs substrate 1 of AlGaAs compound materials. The assembly is then etched down to partway through the n-type GaAs substrate 1, to remove the portion indicated by broken lines in the figure. A stack of the layers 21 to 28 and 30 is then formed by a second crystal growth process of InGaAlP compound materials, in the etched portion. A stripe-shaped oxide film is then formed in a region on each of the first crystal growth side (the left side) and the second crystal growth side (the right side). The left and right ridge-shaped waveguide paths 5/6 and 28/30 are formed by etching. The two-wavelength monolithic semiconductor laser device B can be obtained subsequently by removing the oxide films on the waveguide paths 5/6 and 28/30; forming the p-side electrodes 42, the n-side electrode 41, and the separation groove 43; and then forming the SiO2 films 29C and 29D.
The above-described method of fabrication is characterized in that wet etching is used during the formation of the ridge portions 5 and 28 by etching. The use of such wet etching ensures that the crystals of the ridge portions 5 and 28 are not damaged, in comparison with the use of dry etching. This use of wet etching exposes the (111)A surfaces on the sides of the ridge portions 5 and 28 so that the ridge portions 5 and 28 form quadrilaterals in which the width of the upper edge is narrow and the width of the lower edge is wide. With the device of FIG. 8, the shape of the ridge portions 5 and 28 is such that, if the width of the lower edge is approximately 3 μm and the thickness (height) thereof is approximately 1 μm, the width of the upper edge is approximately 1 μm. In other words, the ratio of the width of the upper edge to the width of the lower edge is approximately 35%.
In the self-sustained pulsation laser of the prior art, the active layers 3 and 24 are made to be thick in order to obtain self-sustained pulsation. In addition, the ridge portions 5 and 28 have a shape such that the width of the upper edge is less than the width of the lower edge. However, the upper limit of the output obtained by the self-sustained pulsation is on the order of 4.5 mW, regardless of the thickness of the active layers 3 and 24 and the technique used to form the shapes of the ridge portions 5 and 28. The lower limit is on the order of 3 mW, although there will be some variation between elements. In other words, self-sustained pulsation can be obtained only within the output region of 3 to 4.5 mW with the self-sustained pulsation laser of the prior art.
That is to say, self-sustained pulsation can be obtained only when the output of the laser beam is weak and the active layers 3 and 24 operate as saturable absorber, as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 6-13709. Thus self-sustained pulsation can be obtained when the absorption in the active layers 3 and 24 exceeds the gain. In such a case, the active layers 3 and 24 of the laser device B of FIG. 8 could be made thicker in order to make it easier for self-sustained pulsation to occur. If the active layers 3 and 24 are made thicker, the confinement of the light into the active layers 3 and 24 will be stronger, making it easier for the active layers 3 and 24 to operate as absorber and thus easier for self-sustained pulsation to occur. More specifically, the thickness of the active layer 3 in the laser for CDs on the left side of the figure should be at least 20 nm. Similarly, the total thickness of the well layers of the active layer 24 in the laser for DVDs should be at least 20 nm. In contrast thereto, to achieve a high output from a high-output laser, the active layer is made thinner so that the confinement of light into the active layer is weakened.
In the laser device shown in FIG. 8, the ridge portions 3 and 28 are shaped so that width of the upper edge of each is no more than 50% of the width of the lower edge thereof. The injection of currents into the active layers 3 and 24 is constrained by the narrowing of the width of the upper edge, the light-generating regions of the active layers 3 and 24 is limited to the central portions under the ridge portions 5 and 28, and the surface area of the portions of the active layers 3 and 24 that produce the high gain is reduced. This is because it is considered in the prior art that absorption by the active layers 3 and 24 can be made more likely to occur and self-sustained pulsation can be made more likely to occur by reducing the surface area of the portions in which gain is high and increasing the surface area of portions in which absorption occurs. And this is because it is easy to form the ridge portions 5 and 28 so that the width of the upper edges thereof are narrow by using wet etching.
As described above, the thicknesses of the active layers 3 and 24 and the shapes of the ridge portions 5 and 28 are manipulated in the self-sustained pulsation laser device of the prior art. However, the upper limit of the output region in which self-sustained pulsation is achieved is restricted to approximately 4.5 mW. But this has been thought to be inevitable, due to the above-described self-sustained pulsation mechanism.